Manufacture of silicon material for crystal contacts



Patented Oct. 14, 1947 MANUFACTURE OF SILICON MATERIAL FOR CRYSTALCONTACTS Charles Eric Ransley, London W. 14, and Kenton,

Sudbury, John Walter Ryde, Stanley Vaughan Williams, England, assignorsto The General Electric Company Limited, London, England No Drawing. Aplication December 10, 1942, Se-

rial No. 468,577. In Great Britain December 7 Claims.

This invention relates to the manufacture of silicon material forcrystal contacts, suitable for use as electric rectifiers and mixers.

It has been proposed in patent application Serial No. 454,290 filedAugust 10, 1942, Patent Number 2,419,561 dated April 29, 1947, tointroduce into the silicon a small quantity (e, g. not more than 2%) ofaluminium and beryllium or a mixture of the two. If this additive isadded to molten silicon and the melt is cooled slowly, it is sometimesfound that the additive is not distributed uniformly in the solidmaterial. When the material is broken up so as to yield the small piecessuitable for crystal contacts some of these pieces may be almost freefrom additive, and therefore relatively unsuitable. This is the morelikely to happen, the larger the amount of the melt. The object of thisinvention is to produce material in which the additive is consistentlydis ributed with greater uniformity.

The probable explanation of the lack of uniformity is that the additivehas a very low solubilit in silicon at atmospheric temperature andtherefore segregates during cooling, the segregation being the morecomplete the slower the cooling. Since the silcon is usually melted invacuo, it is not convenient to quench the melt.

According to the invention, a process of manufacturing semi-conductingmaterial for crystal contacts, comprising the step of melting silicontogether wi h a small amount of additive, for example aluminium orberyllium or a mixture of aluminium and beryllium. and cooling the meltslowlv to produce a solid p oduct. comprises the further step. performedbefore the solid product is div ded into pieces each suitable for asingle contact. of maintainingthe solid product for a con iderab e timeat a temperature great y above atmospheric temperature, but below themelting point of the product. and then quenching the product rapidly.the time and tem era ure o this heat trea ment being chosen so that theadditive is distributed approximately uniformly in the quenched product.In the said further step the product may be heated in air, but of coursea neutral or reducing atmosphere may be used if it is desirable for anyreason; the choice of atmosphere will-only affect an outer skin of theproduct. The term cool slowly in the foregoing statement is correlativeto the term quench rapidly; it implies that the rate of cooling duringthe slow cooling is much less than in the rapid quenchg On theoreticalgrounds it is probable that the minimum temperature at which the heattreat- 2 ment will produce the desired result is that at which theadditive is completely soluble in the silicon. Since the solubility is amaximum at the eutectic temperature, the said minimum temperature cannotbe higher than the eutectic temperature, which is about 575 C. foraluminium and 1090 C. for beryllium. The eutectic temperature of thesystem aluminium-beryl]ium-silicon appears not to be known. We havefound that, when the additive is 3 1% of aluminium, heating at 575-580C. for several hours will produce the desired result. But if the amountof the addi-- tive is less than that corresponding to the eutectic(which will certainly be the case when the additive is beryllium), theminimum temperature may be muchbelow the eutectic temperature; more overthere is no reason why temperatures approaching the minimum should beused. Accordingly these theoretical considerations are of little valueas a guide to practice. Whether the additive is aluminium or berylliumor a mixture of the two (We have found A;% aluminium and [2% berylliumvery suitable), maintaining the product at 1050 C. for one hour and thenquenching in water has been found to produce the desired uniformity; weknow of no reason why any other heat treatment should be used.

In the said application Serial No. 454,290 it was proposed that theadditive should be added in the form of metal mixed with the siliconbefore the melt. However the additive, especially when it is or containsberyllium, may be introduced into the silicon partly or wholly byreaction of the silicon during the melting with, the material of thecrucible in which the melting takes place. The method by which theadditive is introduced into the melt is irrelevant to the invention; andwe desire it to be understood that in the foregoing statement of theinvention and in the appended claims, the phrase melting silicontogether with a small amount of additive is to be interpreted to includeany step in which a mixture of s licon and the additive is maintainedfor an appreciable time in the molten state.

We claim:

1. A process of manufacturing semi-conduct ing material for crystalcontacts, comprising the step of melting silicon together with a smallamount of additive, and cooling the melt slowly to produce a solidproduct, which comprises the further step, performed before the solidproduct is divided into pieces each suitable for a single contact, ofmaintaining the solid product for a considerable time at a temperaturegreatly above atmospheric temperature, but below the melting point ofthe product, and then quenching the product rapidly, the time andtemperature of this heat treatment being chosen so that the additive isdistributed approximately uniformly in the quenched product.

,2. A process of manufacturing semi-conducting material for crystalcontacts, comp-rising the step of melting silicon together with a smallamount of aluminium, and cooling the melt slowly to produce a solidproduct, which comprises the further step, performed before the solidproduct is divided into pieces each suitable for a single contact, ofmaintaining the solid product for a considerable time at a temperaturegreatly above atmospheric temperature, but below the melting point ofthe product, and then quenching the product rapidly, the time andtemperature of this heat treatment being chosen so that the aluminium isdistributed approximately uniformly in the quenched product.

3. A process of manufacturing semi-conducting material for crystalcontacts, comprising the step of melting silicon together with a smallamount of beryllium, and cooling the melt slowly to produce a solidproduct, which comprises the further step, performed before the solidproduct is divided into pieces each suitable for a single contact, ofmaintaining the solid product for a considerable time at a temperaturegreatly above atmospheric temperature, but below the melting point ofthe product, and then quenching the product rapidly, the time andtemperature of this heat treatment being chosen so that the beryllium isdistributed approximately uniformly in the quenched product.

4. A process of manufacturing semi-conducting material for crystalcontacts, comprising the step of melting silicon together with a smallamount of a mixture of aluminium and beryllium, and cooling the meltslowly to produce a solid product, which comprises the further step,performed before the solid product is divided into pieces each suitablefor a single contact, of maintaining the solid product for aconsiderable time at a temperature greatly above atmospherictemperature, but below the melting point of the product, and thenquenching the product rapidly, the time and temperature of this heattreatment being chosen so that the mixture of aluminium.

and beryllium is distributed approximately uniformly in the quenchedproduct.

5. A process of manufacturing semi-conducting material for crystalcontacts, comprising the step of melting silicon together with a smallamount of aluminium, and cooling the melt slowly to produce a solidproduct, which comprises the further step, performed before the solidproduct is divided into pieces each suitable for a single contact, ofmaintaining the solid product for one hour at a temperature of 1050" C.and then quenching the product rapidly.

6. A process of manufacturing semi-conducting material for crystalcontacts, comprising the step of melting silicon together with a smallamount of beryllium, and cooling the melt slowly to produce a solidproduct, which comprises the further step, performed before the solidproduct is divided into pieces each suitable for a single contact, ofmaintaining the solid product for one hour at a temperature of 1050 C.and then quenching the product rapidly.

'7. A process of manufacturing semi-conducting material for crystalcontacts, comprising the step of melting silicon together with a smallamount of a mixture of aluminium and beryllium, and cooling the meltslowly to produce a solid product, which comprises the further step,performed before the solid product is divided into pieces each suitablefor a single contact, of maintaining the solid product for one hour at atemperature of 1050 C. and then quenching the product rapidly.

CHARLES ERIC RANSLEY. JOHN WALTER RYDE. STANLEY VAUGHAN WILLIAMS.

